1. Field of the Invention
The present invention relates to a collision detecting method capable of quickly securely detecting a collision between a driven body driven by means of a servomotor and an obstacle by using an observer.
2. Description of the Related Art
During the operation of a robot, machine tool, etc., a machine operating section, such as a robot arm and a machine table, or a workpiece mounted on the table sometimes may run against an obstacle, e.g., a foreign matter intruded in an operating region of a driven body. In a machine which uses a servomotor as a drive source, in such a case, the servomotor continues to produce a great output torque to move the driven body to a command position even after the driven body, having run against the obstacle, is stopped from moving. In some cases, therefore, machine components including the servomotor may be damaged, or a secondary accident may be caused. Conventionally, in order to avoid such an awkward situation, occurrence of a collision is detected by various methods, and the rotation of the motor is stopped when a collision occurs.
As is generally known, a collision between the driven body and the obstacle may be detected by using a touch sensor mounted on the outer surface of the driven body, for example. Mounting this sensor, however, entails an increase in cost of the machine, and a collision between the driven body and the obstacle in any other position than the operating region of the sensor cannot be detected. In the machine whose driven body is driven by means of the servomotor, moreover, it is a known method to detect occurrence of a collision when a predetermined value is exceeded by a servomotor driving current which increases when the driven body and the obstacle run against each other. It takes a lot of time, however, for the servomotor driving current to actually attain the predetermined value after the occurrence of the collision, so that a detection delay is caused, and the resulting damage is substantial. It is also common knowledge that an alarm may be given after discriminating occurrence of a collision when an unreasonable value exceeding an allowable value is reached by the difference (positional deviation) between the command position and the actual position of the driven body. However, some of the machine components may be damaged before the allowable value is exceeded by the positional deviation after the occurrence of the collision.
Accordingly, a method for quickly detecting occurrence of a collision has been proposed in which the occurrence of the collision is discriminated or detected when a threshold value is exceeded by a differential value of a speed deviation or a differential value of a torque command value (see Japanese Patent Application No. 63-32866). If a collision is caused while a servomotor whose speed command itself assumes a small value is rotating at low speed, however, the speed deviation, which depends on the speed command and the actual speed, increases relatively slowly, so that the differential value of the speed deviation and the differential value of the torque command corresponding thereto are not very large values. In order to detect the occurrence of a collision quickly, therefore, the threshold value should be set at a small value. In this case, the occurrence of a collision is liable to be wrongly discriminated in a normal rotating speed range although no collision is caused.